Stripping process



Patented July 16, 1940 UNITED, STATES STRIPPIN G PROCESS Charles H. Hempel, Manitowoc, Wis, assignor to Heresite & Chemical Company, a corporation of Wisconsin No Drawing. Application November 22, 1939, Serial No. 305,731

2 Claims.

This invention relates to a stripping process and is an improvement upon the process set forth in Patent No. 2,127,469, issued August 16, 1938, upon my application Serial No. 150,213,

5 filed June 24, 1937.

The invention relates primarily to stripping of aluminum or predominantly aluminum alloys which havve been coated with varnishes, lacquers or enamels, especially oil-free, of the phenol-formaldehyde type, particularly when the coatings have been baked. The invention is also applicable to stripping other coated metals, but with less reactive metals than aluminum there are other processes which may also be employed.

Aluminum or aluminum alloy materials coated with such phenol-formaldehyde materials are used to a large extent in certain industries, particularly on buckets and spinning spools in the rayon industry. Such articles after service sufgo fer damage to the coating and are rendered unfit for use. The underlying aluminum, however, is still of value and it is desirable to be able to remove the coating without damaging the aluminum.

In the process described in Patent 2,127,469, a combination of sulphuric and nitric acids was employed to remove the resinous coating without damaging the underlying active metal. The presence of nitric acid is, however, quite danger- 30 ous. The temperature of the bath containing the nitric acid is diflicult to control and it occasionally boils over with resulting considerable danger. Also, heavy fumes are likely to result from the operation.

35 Sulphuric acid by itself, however, will either fail to attack the resinous coating or, if used under conditions suitable for attacking the coating, will also attack the aluminum. It has now been discovered that if sulphuric acid having a specific gravity of not less than approximately 1.83 (66 B.) is heated to a temperature of not less than 180 C. and not more than 190 C., it will attack the resinous coating readily and will not attack the aluminum.

At temperatures below 180 C., the sulphuric acid will not attack the coating, whereas at temperatures above 190 C. it will attack the aluminum.

Sulphuric acid having a specific gravity above 1.83 may be employed within the same temperature range.

As an example of the invention, sulphuric acid having a specific gravity of 1.83 was heated to approximately 185 C. in a cast iron tank. The parts to be stripped are immersed in the solution either slowly enough so that the temperature thereof does not fall below 180 C., or else following preheating of the parts so that they will not cool the solution. The bath is carefully maintained within the range. of 180-190 C. at all times. After about minutes the resinous coating is completely removed.

What I claim as new, and desire to secure by Letters Patent, is:

1. The method of removing a baked phenolformaldehyde coating from a predominantly aluminum base which comprises subjecting the coated base to a bath consisting essentially of sulphuric acid having a specific gravity of at least 1.83, while maintaining the temperature of the bath between 180 and 190 C., and continuing the treatment until the base is bright and clean.

2. The method as set forth in claim 1, in which the coated base is maintained in the bath for approximately 20 minutes.

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